The present invention relates to a digital equalizer and a FM receiver having the digital equalizer, and more particularly to a multipath correcting filter which corrects a multipath signal component present in a signal which is received by an FM receiver, for example.
One problem which is frequently encountered in the reception of FM broadcasts is multipath transmission. The multipath transmission is a phenomenon where a single radio wave or a plurality of radio waves which are reflected and delayed by a mountain or mountains, a building or buildings, or other objects, reach the receiver via two or more paths together with a radio wave directly from the transmitter. After such multiple signals are frequency-modulated, they result in distortion in the receiver and lower the quality of the broadcast which is received. The problem of multipath transmission manifests itself particularly with mobile receivers such as car radio receivers.
In order to solve the multipath transmission problem, it is necessary to obtain the proper radio signal from the signals which have reached the receiver by multipath reception. One scheme is to use an adaptive filter which carries out adaptive signal processing, the adaptive filter being used as a multipath correcting filter. According to the adaptive signal processing effected by the adaptive filter, the mean square of an error or difference between a desired response for the filter and an actual output of the filter, i.e., the mean square error is defined as an evaluating function, and the coefficient of the filter is controlled such that the evaluating function will be minimized.
There is known an application in which an adaptive filter is used as a multipath correcting filter. The multipath correcting filter employs the envelope (amplitude) of an FM signal transmitted from a broadcasting station, as a desired response for the filter, and any error between the desired response and the output of the filter is processed according to the adaptive signal process. Multipath correcting filters which are based on the above concept are disclosed in the following documents:
1. "A New Approach to Multipath Correction of Constant Modulus Signals" written by J. R. Treichler and B. C. Agee, IEEE trans. Vol. ASSP-31, No. 2, pp. 459-471 (1983); and
2. "Multipath Removal from Constant-Envelope Signals With Decision Feedback System" written by Itami and Hadori, Shingakugiho, CAS877-154, pp. 19-24 (1987).
Sufficient multipath correction of FM broadcast signals using the above known multipath correcting filters, however, requires a considerable number of filter coefficients, and, as a result, the amount of the computational filtering process and the computational process for filter coefficient control is enormous.
Another conventional multipath correcting filter normalizes the transfer function of a multiple transmission path with reference to a radio wave transmitted directly from the transmitter (hereinafter referred to as a "direct radio wave", for the computational filtering process. The following documents show this type of multipath correcting filter:
3. "Modified Adaptive FIR Equalizer for Multipath Echo Cancellation in FM Transmission" written by K. D. Kammeyer, R. mann, and W. Tobergte, IEEE J vol. SAC-5, no. 2, pp. 226-237 (1987); and
4. Japanese Laid-Open Patent Publication No. 62(1987)-140527.
Also known is a multipath correcting filter in which the digital filter in the above conventional design comprises a cascaded FIR finite impulse response filter) also known as a "transversal filter").
In actual radio signals, the amplitude of a direct radio wave is unknown, and it is impossible to normalize radio signals applied to the multipath correcting filter. Therefore, it is difficult to determine a reference value. With mobile receivers such as car radio receivers, the intensity of a received direct radio wave varies from place to place, and hence the received multipath signals cannot be normalized according to the amplitude of the direct radio wave. In this case, the amplitude of the direct radio wave itself is used as a reference value, rather than any normalized signal. Accordingly, it is highly difficult to obtain an output signal having a constant amplitude.